Atmospheric corrosion resistant steels have been applied to many actual structures including bridges because of its unique property of preventing rust by rust. There are many cases where a reduction in maintenance management costs is attained by using this steel material with its functions utilized to the full. On the other hand, a problem sometimes arises when the atmospheric corrosion resistant steel is carelessly used in coastal regions and the like where there is a lot of airborne salt. In recent years, in inland regions as well, local formation of abnormal rust due to the spraying of thawing salt is sometimes found.
As is represented by the minimum maintenance bridge concept proposed by the Ministry of Land, Infrastructure and Transport of Japan, future structures including bridges are requested to employ atmospheric corrosion resistant steels, surface treatment technology, structural design methods, and the like which can be actually used reliably for a very long period while achieving a further reduction in maintenance and management costs. Therefore, as a form of 21st century type infrastructure which maintains and develops cost-competitiveness with Asian countries as our country as a whole, rust science study and application technology development related to the atmospheric corrosion resistant steel capable of realizing Life Cycle Cost (“LCC”) minimum are greatly expected.
Against this background, in the Japan Society of Corrosion Engineering, the Rust Science Workshop which supports 21st century infrastructure was organized for four years from 1997. Many specialists enthusiastically discussed the concept of “stable rust” heretofore very confused as basic understanding to use atmospheric corrosion resistant steels more securely and safely.
As a result, the following suggestion has been issued as the opinion of the Society in the 132nd Corrosion Engineering Symposium sponsored by the Japan Society of Corrosion Engineering held on Jun. 25, 2001 (Japan Society of Corrosion Engineering, Rust Science Workshop: Reference materials for the 132nd Corrosion Engineering Symposium “New Development of Rust Science to Realize Minimum Maintenance Bridge Concept”, p 3, Jun. 25, 2001).
“Rust stabilization” of the atmospheric corrosion resistant steel means a state in which the corrosion rate has reduced to the extent that the secular deterioration of the load carrying capacity of a structure is insignificant from an engineering viewpoint (0.01 mm/year or less as a standard).
As provisional interpretation: the stable rust is rust formed when rust on the atmospheric corrosion resistant steel is “stabilized”. However, although the aforementioned state is defined as “rust stabilization”, since the term of stable rust has a strong physical image, it is desirable to withhold the scientific use of this term. As an alternative material term, the term of protective rust is used for rust having a high protective function.
Rust when “stabilized” is characterized in that although a sufficient period (e.g., five years or more) has elapsed, the rust does not grow thick (except a case where traces of exfoliated rust are left).
One of the important messages in this suggestion is the definition of “rust stabilazation” of the atmospheric corrosion resistant steel. Namely, when the industrial material called an atmospheric corrosion resistant steel is used for a structure, the realization of “a rust stabilization state” in which the load carrying capacity of the structure using this material can exist stably over a long period is recognized anew as a higher objective concept than other various arguments about what the matter called stable rust is. Moreover, the development of the technology of predicting the accumulated amount of corrosion of the atmospheric corrosion resistant steel over a long period of time is suggested as one of the most important items in material selection, structural design, and maintenance/management.
Conventionally, among methods for predicting an accumulated corrosion amount of an atmospheric corrosion resistant steel over a long period of time which are generally performed, there is a method of performing an exposure test over a period of approximately ten years in a construction site or under atmospheric environmental conditions similar to those of the construction site, finding a value A and a value B by fitting a secular change of a corrosion loss obtained in this period to a relational expression of (amount of corrosion)=A×(exposure period)B, and calculating a corrosion loss over any given long period of time with these values (see, for example, Public Works Research Institute of the Ministry of Construction, the Kozai Club, Japan Association of Steel Bridge Construction: Collaborative Research Report on Application of Atmospheric corrosion resistant steel to Bridges (XII), p 20, March, 1992).
However, in this predicting method, the exposure test in actual atmospheric environment over a period of approximately ten years is necessary to obtain constant terms, the value A and the value B, and funds, labor, and time are needed before a judgment is made. Hence, a problem that the market competitiveness of a technical business method adopted at present of the atmospheric corrosion resistant steel is weaker than that of concrete structures and the like which compete with the atmospheric corrosion resistant steel is indicated.
As for a flow concerning judgment on the applicability of the atmospheric corrosion resistant steel, flows such as shown in FIG. 1 to FIG. 4 are disclosed in Japanese Patent Laid-open No. 2000-1816 and so on. However, in each flow, only factors which contribute to the amount of corrosion in the usage environment are substantially arranged, and no quantitative criterion for judging the propriety of use of a steel type to be applied based on a predicted corrosion amount in an adaptive environment is proposed or disclosed. Namely, these flows are not effective solutions for a demand for a more quantitative judgment method based on the predicted corrosion amount. Moreover, these flows have a problem that the amount of sulfur oxide and the annual wetness time which are important parameters for the prediction of the corrosion amount are not considered at all.
In the conventional method for predicting long-term corrosion/wear of the atmospheric corrosion resistant steel, exposure test data in the construction site or exposure test data in an atmospheric environment similar to the construction site are indispensable, and to obtain the data, high expenses of test/analysis are needed. Further, regions where unpainted atmospheric corrosion resistant steels are used for road bridges in our country are limited to regions where the amount of airborne salt is 0.05 mdd or less (mdd is a brevity code of mg/dm2day), but in some cases, abnormality does not occur to the atmospheric corrosion resistant steel even under the environmental condition of an airborne salt amount of 0.05 mdd or more. Therefore, there are some cases where an opportunity to reduce the maintenance cost is missed because the applicability is judged only with a single index. Furthermore, when the atmospheric corrosion resistant steel is used beyond the limit of application without sufficient prediction of corrosion/wear behavior, partial abnormal corrosion occurs, which causes unexpected repairing expenses.
As described above, no solution which associates corrosivity of environmental condition with rust stabilization performance of the atmospheric corrosion resistant steel exists, and hence it is said that the application of the atmospheric corrosion resistant steel entails high risks and high returns.